1. Technical Field
The present invention relates to a thermoplastic elastomer composition and a cover for an air bag device. More particularly, the present invention is concerned with a thermoplastic elastomer composition comprising, in specific amount relationships, a mixture of two different specific hydrogenated block copolymers which are different in molecular structure and number average molecular weight, a polyolefin resin having a specific melt flow rate, and optionally a hydrocarbon oil; and also concerned with a cover for use in releasably holding an air bag device, which comprises a molded body formed by molding the thermoplastic elastomer composition.
The thermoplastic elastomer composition of the present invention has excellent softness, heat resistance, low-temperature characteristics, weatherability, mechanical strength, moldability and scratch resistance (resistance of the surface of a molded article to scratching) and, therefore, can be advantageously used as a material for various types of articles, such as parts of automobiles or electrical appliances, toys, miscellaneous goods and the like. Especially, by virtue of the excellent scratch resistance, the elastomer composition can be advantageously used as a material for articles which require maintenance of good appearance. Examples of such articles include interior parts of automobiles, such as an instrument panel, an arm rest, a steering wheel and a horn pad, and exterior parts of automobiles, such as a window molding and a bumper. In addition, it is noted that a cover for use in releasably holding an air bag device, which comprises a molded body formed by molding the above-mentioned thermoplastic elastomer composition, exhibits not only the above-mentioned excellent properties, but also excellent break-opening characteristics in a wide temperature range of from -40 to 90 .degree. C. without occurrence of tearing at unintended portions of the cover and without scattering of broken pieces of the cover. Further, since the thermoplastic resin composition of the present invention exhibits excellent scratch resistance and excellent moldability, it becomes possible to omit cumbersome coating processes, which have been conventionally required for obtaining molded articles having good appearance. Therefore, using the thermoplastic elastomer composition of the present invention, the molded articles can be obtained with high productivity at low cost.
2. Background Art
In the fields of parts for automobiles as well as electrical appliances and medical instruments, miscellaneous goods and the like, in which a vulcanized rubber has conventionally been used as a main material therefor, a recent tendency is noted, wherein thermoplastic elastomers are widely used as materials due to the high productivity which can be achieved by thermoplastic elastomers. Examples of such thermoplastic elastomers include a polyolefin elastomer comprising an ethylene/propylene copolymer and a propylene homopolymer; a polyurethane elastomer; and a soft polyvinyl chloride.
However, when these materials are used for obtaining molded articles, they have disadvantages in that all of the desired characteristics for materials for producing molded articles, such as scratch resistance, softness, moldability, economical efficiency and recyclability, cannot be satisfied simultaneously. For example, polyolefin elastomers are relatively cheap and have excellent weatherability and excellent heat resistance, but they are poor in softness and scratch resistance. Polyurethane elastomers have excellent scratch resistance, but have disadvantages in that they are not only poor in softness and weatherability but also expensive. Soft polyvinyl chloride products are relatively cheap and have excellent weatherability and excellent scratch resistance, but they are poor in softness at low temperatures and in recyclability.
With respect to materials for molded articles, in addition to the above-mentioned materials, some proposals were made, in which attention was directed to an elastomer composition comprising a block copolymer obtained by hydrogenating a vinyl aromatic compound/conjugated diene compound copolymer (hereinafter, frequently referred to simply as "hydrogenated block copolymer"). For example, Unexamined Japanese Patent Application Laid-Open Specification Nos. 50-14742, 52-65551 and 58-206644 disclose elastomer compositions comprising a hydrogenated block copolymer, a rubber softener and a polyolefin resin. However, like polyolefin elastomers, these elastomer compositions have poor scratch resistance.
As an example of applications of thermoplastic elastomers, there can be mentioned a cover for an air bag device. In principle, an air bag system comprises an impact sensor for sensing a collision of a high-speed running object and an air bag device. The air bag device comprises an air bag accommodated in a cavity provided in the high-speed running object at a position opposite to the driver's seat or passenger's seat; an inflator connected to the air bag and capable of generating a gas for expansion of the air bag; a retainer for retaining the air bag accommodated in the cavity, the inflator and the below-mentioned cover; and a cover for covering the opening of the cavity which accommodates therein the air bag, the inflator and the retainer. With respect to the shape of the cover, there is no particular limitation, and the cover can take a box-like shape which can enclose an air bag, a plate-like shape or the like. With respect to the location of the impact sensor in the air bag system, the impact sensor may be located at the bottom of the cavity.
In principle, the air bag system works as follows. In the event of collision, the impact sensor senses the collision and, in turn, an igniting agent in the inflator is ignited electrically or mechanically. The heat produced by the ignition functions to burn a gas generating agent to thereby generate a gas. The generated gas fills the air bag which has been folded up and accommodated in the cavity at a space between the retainer, the inflator and the cover, to thereby expand the air bag. By the pressure of the air bag being expanded, the cover is torn at a predetermined portion and break-opened so as to form an opening, so that the air bag is caused to be instantaneously released through the opening toward the front of the driver (or passenger) and expands. The expanded air bag holds the driver (or passenger) on the seat and functions as a shock absorber so that a collision of the driver (or passenger) with a driving device, an instrument panel or the like, which may lead to an injury of the driver (or passenger), can be prevented. Therefore, it is essential for the cover for an air bag device that, in the event that a collision occurs to cause the gas generator of the air bag system to operate, the cover can be surely break-opened without scattering of broken pieces which may injure the driver (or passenger) so as to make it possible for the air bag to be instantaneously released.
With respect to such a cover for the air bag device, some further proposals were made in, for example, unexamined Japanese Patent Application Laid-Open Specification Nos. 50-127336 and 55-110643. These patent documents disclose a cover which is made of a urethane resin and has a reinforcing net (made mainly of a nylon) embedded therein. Due to the reinforcing nylon net embedded in this type of cover, when the cover is break-opened, prevention can be advantageously achieved with respect to occurrence of unfavorable tearing of the cover at portions other than the predetermined portion, and/or to scattering of broken pieces of the cover. However, the cover having such a reinforcing net embedded therein has disadvantages in productivity, because not only is an additional time required for embedding the net in the cover, but also the embedded net is likely to get out of position.
Unexamined Japanese Patent Application Laid-Open Specification No. 1-202550 discloses a cover for the air bag device, which comprises a surface layer made of a soft material having a JIS K 6301 A hardness of from 30 to 70, and a core layer made of a hard resin having an elasticity and having slits adapted for break-opening the cover. This cover has advantages in that it has a certain degree of stiffness and gives an appropriate soft touch to drivers (or passengers). However, it is necessary to perform a double-layer molding for forming the core layer and the surface layer and, therefore, this cover has a problem in that an expensive double-layer molding machine having two separate injection mechanisms must be used. Further, this type of cover has a disadvantage in that the scratch resistance of the soft material used for forming the surface layer is poor, so that it is necessary to coat the surface of the cover obtained by double-layer molding, leading to an increase in production cost.
Unexamined Japanese Patent Application Laid-Open Specification No. 5-38996 (corresponding to U.S. Pat. No. 5,358,986) discloses a cover for the air bag device, which is obtained by molding a thermoplastic elastomer composition mainly comprising a hydrogenated styrene/conjugated diene block copolymer and having a JIS K 6301 A hardness of from 60 to 85. This type of cover is advantageous in that it gives an appropriate soft touch to drivers (or passengers) and can be used in a wide temperature range. However, this type of cover is disadvantageous in that the scratch resistance of the thermoplastic elastomer composition is poor, so that it is necessary to coat the surface of the cover obtained by injection molding, leading to an increase in production cost.
Unexamined Japanese Patent Application Laid-Open Specification No. 6-200086 discloses a thermoplastic elastomer composition comprising a hydrogenated block copolymer, a polyolefin resin, a paraffin oil and a rubber. The hydrogenated block copolymer used in the elastomer composition comprises a polymer block having a high number average molecular weight, so that the composition tends to have poor moldability. For avoiding such poor moldability, the amounts of paraffin oil and rubber need to be increased; however, this disadvantageously causes the cover for an air bag device (which is obtained by molding such an elastomer composition containing paraffin oil and rubber in large amounts) to have an unsatisfactory scratch resistance.
Unexamined Japanese Patent Application Laid-Open Specification No. 5-171002 discloses an elastomer composition comprising a hydrogenated block copolymer, a polyolefin resin and a hydrocarbon oil. In this elastomer composition, the polyolefin resin is used in a small amount and the hydrocarbon oil is used in a large amount. When such an elastomer composition is molded into a cover for an air bag device, the obtained cover has disadvantages in that it has poor heat resistance, scratch resistance, and break-opening characteristics.